The United States shrimp fishing industry is facing serious economic and environmental problems. Rising costs of shrimp caught by shrimp fishers is making it difficult for this industry to compete with imported shrimp, most of which is raised in ponds. Also, increasing environmental regulations, especially those aimed at reducing the mortality of non-targeted species, is negatively affecting the shrimping industry. The environmental problem relating to non-targeted species arises from the large quantities of non-targeted species of fin fish, as well as turtles, which are caught together with shrimp when using presently available trawling apparatus and often returned dead to the sea. Much of this catch, termed bycatch, is made up of juveniles of commercially valuable species, which are thus wasted. Regulatory actions to reduce this waste problem are rapidly increasing.
Most shrimp are caught by otter trawls towed behind trawlers. The otter trawl was introduced to the shrimp fishing industry in 1912 and has undergone few changes since its inception. Basically, the otter trawl is a large funnel made of mesh netting. Extending from the mouth of the funnel are two mesh extensions, one on either side, that are designed to guide shrimp into the mouth of the net. At the narrow rear end of the net is located the collecting bag, called a codend.
Although the otter trawl was a dramatic advancement to the shrimp fishing industry, nevertheless it has always been troubled with numerous disadvantages: for example, high drag, which increases fuel consumption of the trawler as well as limits towing speed, with the consequent reduction in the amount of area which can be covered per unit of time; difficulty in maintaining the net on the sea bottom at higher speeds (which can be partially corrected by using heavier otter doors, but at the expense of increased drag); the necessity for periodically hauling in the net, emptying it, and recasting it; the need to employ a separate net to determine the presence or absence of shrimp in the water through which the otter net is being drawn; the liability of shrimp and non-targeted species being dead when hauled in; and the difficulty for non-targeted species (bycatch) to escape. As to this last shortcoming, a great effort has been made to solve this problem through the inclusion of turtle exclusion devices (TED) and bycatch reduction devices (BRD), but limited success has been attained. At present, even the most sophisticated otter trawl still suffers from the aforementioned deficiencies because of the inherent limitations of its basic concept.
The solution, then, is to provide a new trawling system and trawl device which will catch more shrimp and kill less bycatch, thus resulting in cost economies and reducing the negative impact on the environment.